An electrical system or power grid comprises energy generating units and energy consuming units or loads. The energy generating units (or in some instances energy transforming units) in an Alternating Current (AC) system typically generates three alternating currents each having a frequency of 50 or 60 Hz and having an offset relative to each other of 120°. To create a highly effective electrical system, the loads in the system should be purely resistive such that voltage and current waveforms are in phase and changing polarity at the same instance in each cycle. With purely resistive impedance all energy generated in the energy generating units will be turned into a useful and desired energy form at the load. However, in most cases, the impedance contains an inductive or capacitive component, which means that the current consumed is not always in phase with the supplied voltage of the alternating currents. The inductive or capacitive components stores energy temporarily in electric or magnetic fields which is then returned to the power grid a fraction of a second later in the cycle. This process results in a time difference between the current and voltage waveforms creating nonproductive power which increases the current. Furthermore, many electrical apparatus comprises active components that have a dynamically varying load, i.e. the load varies with time, such as electrical apparatus converting electrical power to mechanical work.
In a normal AC electric system, the voltage varies sinusoidally at a specific frequency, usually 50 or 60 hertz, known as utility frequency. A linear load, even if it comprises inductive or capacitive components, does not change the shape of the waveform of the current. However, the increased use of pulse controlled apparatuses such as rectifiers, variable frequency drives or arc discharge devices, such as a fluorescent lamps, electric welding machines, or arc furnaces introduces non-linear currents into the power grid. Since currents in these systems are interrupted by a switching action, the current contains frequency components that are multiples of the utility frequency, known as harmonics. As nonlinear currents flow through an electrical system and the distribution-transmission lines, additional voltage distortions are produced due to the impedance of the loads and creates a current waveform which can become quite complex, depending on the type of load and its interaction with other components of the system.
The harmonics are troublesome in many ways; one example is that electric motors experience hysteresis loss caused by eddy currents set up in the iron core of the motor, which are proportional to the frequency of the current. Since the harmonic frequencies are higher, they produce more core loss in a motor than the utility frequency, which results in increased heating of the motor core, which in turn shortens the life of the motor. Other problems with harmonics include overheating of cables and transformers, damage to sensitive equipment and tripping of circuit breakers.
Active filters capable of reducing the inductive and/or capacitive components and harmonics are known in the art, for example in U.S. Pat. No. 7,245,045 to Ström et al., which is hereby incorporated by reference. An active filter is in principle a microprocessor controlled amplifier which is connected to the power grid, and which is arranged to sense and compensate the load's current consumption with regards to frequencies which would not exist if the load was purely resistive.
Typically, an active filter comprises a main circuit with one or a series of fast switches for each phase and each switch is connected to a power source, such as a Direct Current (DC) power source which can accumulate electrical energy. The power grid's current provision and the load's current consumption are measured, and using Pulse Width Modulation (PWM) a compensating current is distributed to the power system by means of the switches.
The current flowing in the electrical system is measured by means of a current measurement unit, after which it is transferred to a computing unit that typically transforms the measured current into a digital frequency domain signal using Fast Fourier Transformation (FFT). The transformed signal is used to create an inverted digital signal which is distributed to the electrical system as a compensating current by means of Pulse Width Modulation (PWM).
The active filters of the art comprise a filter circuit arranged in order to reduce disturbances on the power grid generated by the active filter. Since the active filter in principle is an amplifier, and since the filter circuit comprises an inductor and a capacitor it may form a resonant circuit together with the electrical system which amplifies harmonic frequencies when resonance occurs. Resonance of the power grid may be detrimental by causing unwanted sustained and transient oscillations, which in turn may cause performance degradation. The power grid resonance at the resonant frequency may introduce substantial voltage fluctuation. The effects of resonance in an electrical system may be progressively worsening as higher frequencies are generated. Especially critical for creating resonant behavior is harmonics of 7:th and 11:th order.
Active filters in the art measures the current flowing between the AC power source and a power consuming device. Since the current flowing in the conductors of the electrical system is a result of the voltage in the power source and impedance of the consuming device, in accordance with Ohm's law I=U/R, the measurement of the current will be delayed in relation to fluctuations of the voltage level. On top of that, active filters use an A/D conversion based on Fast Fourier Transform (FFT) which requires relatively computing intense calculations in the microprocessor before a compensating current can be generated. Since the process of creating a compensating current is much too slow for handling resonance, the response of active filters in the art to resonant behavior in the electrical system has been to shut down the filter and deal with the frequencies generating the resonance by adding components to the electrical system.